ABB RELION 620 Series Technical Manual page 186

Section 4
Protection functions
180
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Figure 83: Overcurrent protection of parallel operating transformers
Closed ring network topology
The closed ring network topology is used in applications where electricity distribution for
the consumers is secured during network fault situations. The power is fed at least from
two directions which means that the current direction can be varied. The time grading
between the network level stages is challenging without unnecessary delays in the time
settings. In this case, it is practical to use the directional overcurrent protection relays to
achieve a selective protection scheme. Directional overcurrent functions can be used in
closed ring applications. The arrows define the operating direction of the directional
functionality. The double arrows define the non-directional functionality where faults can
be detected in both directions.
1MAC504801-IB E
620 series ANSI
Technical Manual